This invention relates to image scanning devices in general and more specifically to apparatus for directing light reflected from an illuminated object onto a photosensor.
Optical scanners produce data signals representative of an object or document being scanned by projecting an image of the document onto an optical photosensor. The electrical signals produced by the optical photosensor may then be digitized and processed as necessary to produce an image of the scanned object on a suitable display device, such as, for example, the display of a personal computer. If the object being scanned is text, then the data signals may be converted into text data by a suitable optical character recognition (OCR) program or device.
Most optical scanners use illumination and optical systems to illuminate the object and focus a small area of the illuminated object, usually referred to as a "scan line," onto the optical photosensor. For example, an optical scanner for scanning written documents may utilize a narrow scan line with a length corresponding to the maximum document width to be scanned, e.g., 9 inches or so. The entire object is then scanned by sweeping the illuminated scan line across the object, either by moving the object with respect to the illumination and optical assemblies or by moving the illumination and optical assemblies relative to the object.
A typical scanner optical system will include a slit aperture and lens assembly to focus the image of the scan line onto the surface of the photosensor. Depending on the particular design, the scanner optical system may also include a plurality of mirrors to "fold" the path of the light beam, thus allowing the optical system to be conveniently mounted within a relatively small enclosure. In order to allow a smaller photosensor array to be used, most optical systems also reduce the size of the image of the scan line that is focused onto the surface of the photosensor. For example, many optical systems have a lens reduction ratio of about 8:1, which reduces the size of the image of the scan line by a factor of about 8.
The most common type of photosensor device used in optical scanners is the charge coupled device or CCD. A CCD may comprise a large number of light sensitive cells or "pixels," each of which collects or accumulates an electrical charge in response to light. Since the size of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, a CCD may be used to detect light and dark spots on an image focused thereon. In a typical scanner application, the charge accumulated in each of the CCD cells or pixels is measured and then discharged at regular intervals known as sampling intervals, which may be about 5 milliseconds or so for a typical scanner.
In most optical scanner applications using a long, narrow scan line each of the individual pixels in the CCD are arranged end-to-end, thus forming a linear array. Each pixel in the CCD array thus corresponds to a related pixel portion of the elongate scan line. The individual pixels in the linear photosensor array are generally aligned along a "cross" direction, i.e., a direction perpendicular to the direction of movement of the illuminated scan line across the object. The direction of movement of the illuminated scan line across the object is known as the "scan direction." Each pixel of the linear photosensor array thus has a length measured in the cross direction and a width measured in the scan direction. In most CCD arrays the length and width of the pixels are equal, typically being about 8 microns or so in each dimension.
As mentioned above, each pixel in the CCD array corresponds to a related pixel portion of the elongate scan line on the object. To avoid confusion, the corresponding pixel portion on the elongate scan line will be referred to herein as an "native object pixel." A native object pixel has dimensions equal to the dimensions of the corresponding pixel on the linear photosensor array multiplied by the lens reduction ratio of the optical system. For example, in a scanner having a CCD pixel size of 8 microns by 8 microns and a lens reduction ratio of 8:1, the size of the native object pixels will be about 64 microns by 64 microns. Also, the linear array of native object pixels that corresponds to the linear array of CCD pixels will be referred to herein as a "native scan line."
While optical scanners of the type described above are widely used, they are not without their disadvantages. For example, the optical systems used in such scanners generally employ several optical elements which may be expensive to manufacture and difficult to align. For example, the lens assembly used to focus the image of the illuminated scan line onto the surface of the photodetector may represent a significant portion of the overall cost of the scanner device. Of course, if low cost lens assemblies are used, the cost savings usually comes at the expense of increased image aberrations, such as astigmatism, coma, etc., which have the effect of decreasing the overall image quality. Many optical scanners also use mirror assemblies to fold the path of the light beam. While such mirror assemblies have the advantage of allowing the optical system to be mounted within a relatively small enclosure, they may be difficult to align and may impose strict geometrical relationships between the various components of the scanner.
Another disadvantage associated with the image scanning devices of the type described above is that the linear CCD arrays are relatively expensive, and only a few different configurations are currently available from manufacturers. Further, since it is not practical to construct linear CCDs with lengths even approaching the length of a typical scan line, which may be about 9 inches, the optical system must have a fairly high lens reduction ratio in order to reduce the length of the image of the illuminated scan line to allow reasonably short detector lengths, on the order of about 1 inch or so. Unfortunately, however, high lens reduction ratios tend to reduce the native resolution of the scanner. For example, an optical scanner having a CCD with a pixel size of 8.times.8microns and a lens reduction ratio of 8 will have native object pixels of about 64.times.64 microns. Therefore, the lens reduction ratio imposes a limit on the maximum native resolution that can be achieved by a given CCD photodetector.
Consequently, there remains a need for an optical scanner having a simplified system for directing an image of the illuminated scan line onto the surface of a photodetector. Ideally, such a simplified optical system would eliminate the need for the slit aperture and lens assemblies, as well as the need to resort to relatively complex and difficult to align mirror systems. Additional utility could be realized if the effective lens reduction ratio could be decreased, thus decreasing the size of the native object pixels and increasing the native resolution of the scanner.